Lord of Coastal Waters, Maia of Ulmo, he who enjoys perhaps overmuch the winds of Manwe which allows him to build the wild waves in which he laughs. He is friend and instructor of the Teleri elves, they of the great swan boats. At one time Osse was brought into the service of Melkor but, with the help of Uinen as requested by Aule, became one of the very rare ones who pulled away from such service, returning to receive the pardon of Ulmo and once more being on the side of the Valar. Mariners and coastal dwellers love Osse, but do not trust him. Osse and Uinen, his wife, are the Maiar best known to those who live in Arda.
Reference: The Silmarillion